Always add 360 , PS3 , or PC to clarify which system the bug appears on.

PC 360 Using the spell Detect Dead or the Shout Aura Whisper will reveal all of the Reaper's minions inside the floor. By standing above one of these minions and attacking, the Reaper and his minions will rise and attack the player, at which point they can be fought and killed without placing or gathering any of the Reaper Gem Fragments. Another way, not requiring the Detect Dead spell, is by walking to the far side of the room from the door. The ghostly smoke effect of the reaper can be seen through the pile of bones he is standing in, about where his head would be. If the Dragonborn goes up to him and tries to select him as if to initiate dialog, he will step forward and his minions will rise.

360 It is possible for the creature to attack without using the required items to summon it.

360 It is possible to force the Reaper and his minions to attack by using area of affect attacks on the bone piles. A simple Unrelenting Force shout works or a Simple Fire Rune.